The present invention relates to supported metal catalysts.
It is known to employ metal alkoxides in the preparation of solid catalysts. The reasons for employing the metal alkoxides vary considerably, as can be seen from the following description of the use of catalysts in the literature. For example, U.S. Pat. No. 3,873,469 discloses the preparation of metal catalysts for the purification of automobile exhaust gases. The catalysts are prepared by impregnating a precious metal such as platinum, palladium, rhodium, iridium, ruthenium or mixtures thereof onto a monolithic honeycomb structure. The honeycomb structure is composed of refractory compounds of low surface area. The chemically inert and refractory nature of this honeycomb is extremely important, as it should not undergo any drastic transformations during the extreme conditions encountered in typical automobile exhaust systems. However, impregnation of precious metals onto such a support surface to obtain a thermally stable catalyst having reasonably high metal dispersion is practically impossible. This difficulty is overcome in the teaching of said patent by employing a metal alkoxide to "wash coat" the low surface area honeycomb to obtain a honeycomb having higher surface area. The higher surface area support is suitable for catalyst preparation.
Similiarly, U.S. Pat. No. 4,076,792 discloses the use of metal alkoxides to prepare layered support coatings on monolithic honeycombs to make catalysts having both platinum and rhodium deposited on a wash coated honeycomb. The wash coating method comprises the impregnation of the selected support structure with the lower alkoxides of metals followed by the in situ hydrolysis of the metal alkoxides to form an adherent coating of hydrous metal oxides. The coated support structure may then be fired to convert the hydrous metal oxides to an oxide support coating of very high surface area and good porosity.
The use of metal alkoxides to supply alkali metal ions in a nonaqueous form is described by R. Hombek, J. Kijenski, and S. Malinowski (Warsaw, Poland) in a paper presented at the Second International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, held at Louvain-la-Neuve, Belgium, on Sept. 4-7, 1978 (Proceedings published as Studies in Surface Science and Catalysis, Vol. 3, by Elsevier Scientific Publishing Company, Amsterdam/Oxford/New York, 1979; pages 595-603). Many industrial catalysts, especially for dehydrogenation and cracking processes, are modified by the controlled dosing of alkali metal ions to suppress the unwanted strong acidity of the catalyst carrier or support. The new alkali impregnation procedure, using alkali metal alkoxides in alcoholic solutions, avoids the reaction of water with dehydrated alumina support surfaces; this is particularly important because the water causes considerable changes in the properties of catalysts prepared by the aqueous alkali-hydroxide addition method.
Still another development reported by M. Glinski and J. Kijenski (same Polish group as above) is the method of impregnation with vanadyl tri-isobutoxide to prepare vanadium-alumina and vanadium-silica catalyst systems. This is described by the Polish authors in their paper presented at the Third International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysis, Vol. 16, 1983; pages 553-561. As in the cas of impregnation with alkali-metal alkoxides, the application of vanadyl alkoxide in a non-aqueous medium avoids the secondary effects caused by the interaction of water with the dehydrated alumina or silica surface. Following the alkoxide impregnation, the resultant product was calcined in a stream of dry air at 573.degree. K. for 3 hours to obtain the respective vanadium-alumina or vanadium-silica catalysts. The catalysts can further be reduced if necessary.
The principle of bringing the catalytically active material or materials in the form of an alkoxide precursor, followed by hydrolysis or thermal decomposition to obtain the final supported metal oxide or metal catalyst, is also the basis of the invention of U.S. Pat. No. 4,400,306. This method of catalyst preparation comprises: impregnating a preformed catalyst support with a solution of an alkoxide of at least one metal selected from V, Mo, Sb, Cu, Nb, Ta, Zn, Zr, B and mixtures thereof; contacting the impregnated support with a solution of at least one additional catalyst component to form the catalyst in situ, and drying or calcining the thus formed catalyst. The preformed catalyst support in such cases can be of different types and shapes, e.g., non-porous or microporous fluidizable powders, pellets or tablets, extrudates, monoliths and similar forms. The patent teaches (U.S. Pat. No. 4,400,306; column 3, lines 22-31) that "the use of metal alkoxides in the impregnation step is advantageous because the alkoxide reagent represents an extremely pure source of the metal and metal oxide reagents, unlike water-soluble salts such as the alkali metal or sulfur-containing salts which carry possibly unwanted counter-ions into the support material for incorporation into the catalyst. Upon heating or calcining, the alcohol adduct of the alkoxide is driven off or oxidized to form a metal oxide species." This use of the metal alkoxide as an extremely pure source of the metal is also one of the motives for using it to wash coat monolith honeycombs, as described in U.S. Pat. No. 3,873,469, cited hereinabove. Still another embodiment of the invention described in U.S. Pat. No. 4,400,306 consists of "choosing particular alkoxy derivatives of reducible metal ions" so that an in situ reduction of the metal can be effected (column 3, lines 57-59).
The metal alkoxide impregantion step can also be repeated to increase the amount of metal (or metal oxide) deposited within the support. After the impregnation of one metal alkoxide catalyst component, additional catalyst components can be added in the form of metal alkoxide solutions or solutions of other catalyst component compounds. Catalysts prepared in this way are suitable for fixed-bed or fluid-bed catalytic processes, particularly for oxidation processes like the conversion of C.sub.4 hydrocarbons to maleic anhydride (cf. U.S. Pat. No. 4,455,434).
To summarize, metal alkoxides have been used in the prior art for the manufacture of solid catalysts for the following distinct purposes and with the following specific characteristics:
1. to wash coat a monolithic honeycomb, to give the inert refractory substrate higher surface area and porosity for subsequent metal impregnation (U.S. Pat. No. 3,873,469; U.S. Pat. No. 4,076,792);
2. to bring alkali metal ions in a water-free system to catalysts to neutralize the unwanted strong acidity of the catalysts (Paper of Hombek et al., 1978, supra);
3. to impregnate vanadium alkoxides on silica or alumina to prepare supported vanadium catalysts. The alkoxide is used as a water-free medium to prevent side effects to the catalyst support which would otherwise result from using an aqueous solution of a vanadium salt (Glinski and Kijenski, 1982, supra);
4. use of the specific alkoxide of the catalytically active metal, as alkoxides are generally purer than the corresponding metal salts, hence trace impurities can be avoided if metal alkoxides are used instead of metal salt solutions (U.S. Pat. Nos. 4,400,306 and 4,455,434).
Catalytic metals play an important role in heterogeneous catalysis. The catalytic metals typically are employed on various support materials, as only the surface of a metal particle can participate in a catalytic process. Many people have proposed various solutions to the long-standing problem of how to disperse catalytic metals more efficiently on the surface of a support material. However, the art has not recognized the present invention as being an improved solution to the problem.